Both the spleen focus-forming virus (SFFV) and the Friend murine leukemia virus (F-MuLV) induce erythroleukemia in susceptible strains of mice. Two strains of SFFV, having different biological effects, exist: a polycythemia-inducing strain (FVP) and a mild anemia-inducing strain (FVA). Both FVP and FVA strains of SFFV encode similar enevelope-related proteins, gp52(env), but differ in the post-translational processing of these protein. The gp52(env) of FVP-SFFV is processed to a 65,000 dalton, galactose-containing protein that eventually appears on the cell surface. In contrast, the majority of the gp52(env) encoded by FVA-SFFV is not further processed to a higher molecular weight, galactose-containing form and cannot be detected on the cell surface. The Friend MuLV induces in newborn mice an erythroproliferative disease associated with severe anemia. Studies have indicated that Friend MCF virus is a crucial intermediate in the induction of this disease and that certain mouse strains endogenously express a novel envelope glycoprotein, related to MCF gp70, which conferns resistance to this disease. Studies are presently being carried out to determine how spleen focusforming and Friend MCF viruses interferee with erythropoiesis and why certain strains of mice are resistant to disease induced by these viruses.